Carbonic diesters are compounds of value as automotive gas additives and organic solvents or as reactants, replacing phosgene, in the production of various carbonates, carbamates, urethanes and fine chemicals such as drugs and agrochemicals.
For the commercial production of a carbonic diester, generally the corresponding alcohol is allowed to react with phosgene. However, this known technology demands the use of phosgene having a great toxic potential and, moreover, the reaction of the alcohol with phosgene gives rise to a large quantity of hydrochloric acid which is a highly corrosive substance.
Therefore, a technology has been proposed for producing a carbonic diester without the use of phosgene which comprises allowing the corresponding alcohol to react with carbon monoxide and oxygen in a liquid phase in the presence of a catalyst. The catalyst used for this purpose can be classified into two major categories, i.e. the palladium catalyst including a compound of palladium as the main catalyst component and the copper catalyst including a compound of copper as the main catalyst component.
The reaction using the palladium catalyst is described in Japanese Patent Publication Nos. 8816/1986 and 43338/1986. According to this technology, a palladium compound as the main catalyst component is used in combination with a copper compound and an alkali metal compound. Palladium compounds are advantageous in that they are so active that the reaction proceeds even at a low carbon monoxide partial pressure but has the drawback of giving rise to oxalic acid as a by-product.
The reaction conducted in the presence of a copper catalyst is described in Japanese Patent Publication No. 8020/1981. Copper catalysts are simple in composition and do not give rise to the by-product oxalic acid but since they are less active than palladium catalysts, these catalysts must be used in a large quantity and, moreover, the reaction must be conducted at a high carbon monoxide partial pressure.
However, since the reaction system containing such a catalyst, irrespective of whether it is a palladium catalyst or a copper catalyst, is highly corrosive, the reaction must be conducted in a pressure-resistant reactor having an anticorrosive lining made of e.g. glass or a baked-on type enamel. However, since there is an upper limit to the size of a pressure-resistant reactor having such an anticorrosive lining that can be fabricated, it is difficult to produce a carbonic diester in a liquid phase containing such a catalyst on a commercial scale.
To obviate this corrosion problem associated with a liquid-phase reaction, a technology has been proposed for producing a carbonic diester which comprises allowing the corresponding alcohol to react with carbon monoxide and oxygen in a gas phase in the presence of a solid catalyst. For example, WO87/07601 discloses a production process which comprises allowing all the reactants to react in a gas phase using a catalyst comprising a metal halide supported on a solid support by an impregnation technique. This production process involves a low risk of corrosion and appears to be suited for mass production.
However, as far as the production of dimethyl carbonate starting with methanol is concerned, whereas the liquid-phase reaction provides for a methanol-based selectivity of more than 95% for dimethyl carbonate, the above gas-phase reaction provides only for a low selectivity. Thus, it is disclosed in Example 1 of the above publication that when a feed gas composed of 64.9% carbon monoxide, 10.8% oxygen and 24.3% methanol was introduced through a cupric chloride-on-carbon catalyst bed at a temperature of 115.degree. C. and a pressure of 20 atm, the objective compound was obtained with a selectivity of 80% based on methanol.
Furthermore, WO90/15791 discloses a production process which comprises allowing an alcohol to react with carbon monoxide and oxygen in a gas phase in the presence of a catalyst comprising a copper-tertiary organophosphorus complex supported on activated carbon. In the Examples of this patent literature, the reaction was invariably conducted by feeding a reactant gas mixture of 48.6% carbon monoxide, 2.8% oxygen and 48.6% methanol and the reaction was conducted at a temperature of 150.degree. C. and either at atmospheric pressure or at 6.8 atm. However, according to the check experiments performed by the inventors of the present invention, the selectivities based on methanol under these conditions were in the range of about 80 to 85%.
In these technologies for producing a carbonic diester by gas-phase reaction, the selectivity of the reaction for the objective diester is low and this means not only an increased material cost, viz. the cost of alcohol, but also the need for separation of by-products and these drawbacks add up to a considerable disadvantage in the mass production of a carbonic diester.